FIELD OF THE INVENTION
This invention relates to the art of catalytic cracking of hydrocarbons, and in particular to methods of inhibiting on zeolite catalysts the detrimental effects of contamination by metals, particularly nickel, which are contained in the hydrocarbon feedstock.
Major metal contaminants that are found in Fluid Catalytic Cracker (FCC) feedstocks include nickel, vanadium, iron, copper and occasionally other heavy metals. The problems associated with metal contamination, particularly nickel, during the catalytic cracking of hydrocarbons to yield light distillates such as gasoline are documented in Oil & Gas Journal of July 6, 1981 on pages 103-111 and of Oct. 31, 1983 on pages 128-134. The problems associated with vanadium metal contamination are described in U.S. Pat. No. 4,432,890 and German Pat. No. 3,634,304. The invention herein represents an innovation and improvement over those processes set forth and claimed in U.S. Pat. No. 4,432,890 and German Pat. No. 3,634,304.
It is well known in the art that nickel significantly increases hydrogen and coke and can cause decreases in catalyst activity. Vanadium primarily decreases activity and desirable gasoline selectivity by attacking and destroying the zeolite catalytic sites. Its effect on the activity is about four times greater than that of nickel. Vanadium also increases hydrogen and coke, but at only about one fourth the rate of nickel.
The reducing atmosphere of hydrogen and carbon monoxide in the cracking zone reduces the nickel and vanadium to lower valence states. The nickel is an active dehydrogenating agent under these circumstances, increasing hydrogen and coke which also leads to a small decrease in conversion activity.
Vanadium has been shown to destroy active catalytic sites by the movement of the volatile vanadium pentoxide through the catalyst structure. Lower oxides of vanadium are not volatile and are not implicated in the destruction of catalyst activity. In the cracking zone, lower oxides of vanadium will be present and vanadium pentoxide will be absent. Thus in the cracking zone, fresh vanadium from the feedstock will not reduce activity. When the lower valence vanadium compounds enter the regenerator where oxygen is present to combust the coke, the vanadium compounds are oxidized to vanadium pentoxide which then can migrate to active sites and destroy the active sites, leading to a large reduction in activity and selectivity, particularly gasoline.
An increase in hydrogen and coke due to contaminant metals translates to a decrease in yields of desirable products such as gasoline and light gases (propane/butanes). Also, increases in hydrogen yield require extensive processing to separate the cracked products and can result in operation and/or compressor limitations.
While the coke that is produced during the catalytic cracking process is used to keep the unit in heat balance, increases in coke yields mean increased temperatures in the regenerator which can damage catalysts by destroying the zeolitic structures and thus decrease activity.
As activity is destroyed by contaminant metals, conversion can be increased by changing the catalyst to oil ratio or by increasing the cracking temperature, but coke and hydrogen will also be increased in either case. For best efficiency in a FCC unit, the activity should be kept at a constant level.
However, as vandium is deposited on the catalyst over and above about a 3,000 ppm level, significant decreases in activity occur. Passivators have been used to offset the detrimental effects of nickel and of vanadium.
Numerous passivating agents have been taught and claimed in various patents for nickel. Some examples include antimony in U.S. Pat. Nos. 3,711,422, 4,025,458, 4,111,845, and sundry others; bismuth in U.S. Pat. Nos. 3,977,963 and 4,141,858; tin in combination with antimony in U.S. Pat. No. 4,255,287; germanium in U.S. Pat. No. 4,334,979; gallium in U.S. Pat. No. 4,377,504, tellurium in U.S. Pat. No. 4,169,042; indium in U.S. Pat. No. 4,208,302; thallium in U.S. Pat. No. 4,238,367; manganese in U.S. Pat. No. 3,977,963; aluminum in U.S. Pat. No. 4,289,608; zinc in U.S. Pat. No. 4,363,720; lithium in U.S. Pat. No. 4,364,847; barium in U.S. Pat. No. 4,377,494; phosphorus in U.S. Pat. No. 4,430,199; titanium and zirconium in U.S. Pat. No. 4,437,981; silicon in U.S. Pat. No. 4,319,983; tungsten in U.S. Pat. No. 4,290,919; and boron in U.S. Pat. No. 4,295,955.
Examples of vanadium passivating agents are fewer, but include tin in U.S. Pat. No. 4,101,417 and 4,601,815; titanium, zirconium, manganese, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanides, rare earths, actinides, hafnium, tantalum, nickel, indium, bismuth, and tellurium in U.S. Pat. Nos. 4,432,890 and 4,513,093; yttrium, lanthanum, cerium and the other rare earths in German 3,634,304.
In general, the passivating agents have been added to the catalyst during manufacture, to the catalyst after manufacture by impregnation, to the feedstock before or during processing, to the regenerator, and/or any combination of the above methods.